It is well-known that vinyl aromatic compounds, such as styrene, .alpha.-methylstyrene and other substituted vinyl benzenes, have a strong tendency to polymerize when subjected to elevated temperatures. Since vinyl aromatic compounds produced by common industrial methods contain by-products and impurities, these compounds must be subjected to separation and purification processes in order to be suitable for further industrial applications. Such separation and purification is generally accomplished by distillation techniques.
To prevent premature polymerization of vinyl aromatic monomers during the distillation purification process, various compounds have been used as polymerization inhibitors. Sulfur was widely employed in the past to inhibit polymerization of vinyl aromatic compounds. However in recent times, many chemical compounds have been disclosed or developed as substitutes for sulfur in polymerization inhibiting applications. These compounds have varying degrees of success for industrial use in the distillation process.
In a typical distillation process for a vinyl aromatic compound using a polymerization inhibitor, the mixture containing the vinyl aromatic compound to be distilled is generally contacted with the polymerization inhibitor before being subjected to distillation conditions in the distillation apparatus. It remains a significant problem still that the amount of polymer formed in the distillation system and in the high purity product recovered therefrom is substantially higher than desired. Still worse, occasionally, complete polymerization of the vinyl aromatic compound occurs in the distillation system causing considerable economic loss. A typical distillation system is described in detail in U.S. Pat. Nos. 4,252,615 and 4,341,600, the relevant parts of which are incorporated herein by reference.
U.S. Pat. No. 3,733,326 discloses the polymerization inhibition of vinyl monomers by free radical precursors. Soviet Patent No. 1,027,150 discloses the stabilization of styrene by using nitroxyl radical. Soviet Patent No. 1,139,722 discloses the use of a bis-nitroxyl radical as the thermal polymerization inhibitor for styrene. Japanese Hei 1-165534 discloses the use of 1-piperidyloxy derivatives as polymerization inhibitors for styrene. Soviet Patent No. 1,558,888 discloses the polymerization inhibition of styrene by a nitroxyl radical.
U.S. Pat. No. 4,086,147 discloses a process using 2-nitro-p-cresol as a polymerization inhibitor. U.S. Pat. Nos. 4,105,506 and 4,252,615 disclose a process using 2,6-dinitro-p-cresol as a polymerization inhibitor. U.S. Pat. Nos. 4,132,602 and 4,132,603 disclose the use of a halogenated aromatic nitro compound as a polymerization inhibitor for use during the distillation of vinyl aromatic compounds. However, these aromatic nitro compounds have relatively weak activity, and thus must be used at fairly high concentrations, especially at higher distillation temperatures. Considering the relatively high toxicity for human exposure, these aromatic nitro compounds cannot be regarded as acceptable agents for inhibiting polymerization.
In addition, U.S. Pat. Nos. 3,988,212 and 4,341,600 disclose the use of N-nitrosodiphenylamine combined with dinitro-cresol derivatives for inhibiting the polymerization of vinyl aromatic compounds under vacuum distillation conditions. U.S. Pat. No. 4,466,904 discloses the use of phenothiazine, 4-tert-butylcatechol and 2,6-dinitro-p-cresol as a polymerization inhibitor system in the presence of oxygen during heating of vinyl aromatic compounds. U.S. Pat. No. 4,468,343 discloses a composition and a process for utilizing 2,6-dinitro-p-cresol and either a phenylenediamine or 4-tert-butylcatechol in the presence of oxygen to prevent the polymerization of vinyl aromatic compounds during heating. European patent application 240,297 A1 teaches the use of a substituted hydroxylamine and a dinitrophenol to inhibit the polymerization of a vinyl aromatic compound at elevated temperatures in a distillation process. However, the effectiveness of said systems are oxygen dependent. This results in inconsistent inhibition due to an inconsistent distribution of air throughout the distillation column and raises the possibility of an increased explosion safety hazard. Accordingly, there exists a strong need for a stable polymerization inhibitor system which will effectively and safely prevent the polymerization of vinyl aromatic compounds during distillation and purification processes, particularly if air is absent.